1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus.
2. Description of the Related Art
As has been known, an X-ray computed tomographic apparatus is an apparatus that computes a spatial distribution of CT numbers on the basis of a quantity of X-rays absorbed in a subject's body. A CT number is defined as an X-ray absorption coefficient of tissues, such as organs, indicating a relative value (CT number) with respect to an X-ray absorption coefficient of water as the reference substance.
A reconstructed image contains image noises. A degree of image noises is typically given as a standard deviation of CT numbers within an image of a homogeneous phantom. In order to make a diagnosis by observing a reconstructed image, it is necessary to determine, for example, whether a small shadow on the image is a noise or a tumor. To this end, consideration must be given to an image SD (Standard Deviation) of the image being observed.
The image SD has strong tendency to depend on a transmission dose of X-rays, which is chiefly determined by a relation between a tube current and the subject.
The actual relation between the tube current and the image SD, however, is not a simple one-to-one correspondence, and varies with the body constitution of the subject and many other factors. It is therefore difficult to infer an image SD exactly, and hence to correct the tube current value adequately in response to the image SD. Also, in the case of helical scan, the tissue structure and the body thickness of the subject vary from region to region, and so does a transmission dose, which causes the image SD to vary with the reconstruction positions. This makes it difficult to read the image diagnostically or improves the image SD more than necessary for a diagnosis to be made.